Site author Richard Steane
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Control of heart rate

The main tissue in the mammalian heart is muscle. Heart muscle - cardiac muscle - differs in several ways from other muscle in the body (voluntary and involuntary- AKA striped and unstriped).

Its contractions are myogenic i.e. initiated by the muscle itself, although external stimulation may increase or decrease their frequency. The control of the rate of heartbeat has two components: receptors in different parts of the body and associated nervous pathways, and specialised structures within the heart which interact with the main muscular sections - atria and ventricles.

Hypoxia is low levels of oxygen in the tissues
Hypoxemia is low levels of oxygen in the blood
Hypercapnia is elevated carbon dioxide (CO2) levels in the blood
The rate of heartbeat needs to be varied so as to provide the body - especially muscle tissue - with substances it needs - glucose and oxygen - and to remove waste substances - mainly carbon dioxide.

Cardiac myocytes - muscle cells - contract independently.

Receptors in the circulatory system

Receptors continuously monitor conditions within the body, and so they produce a continuous stream of nervous impulses. The frequency of these impulses can vary as conditions change.

Chemoreceptors detect changes in the blood as a result of increased respiration.
Peripheral chemoreceptors are located in the carotid and aortic bodies (in the carotid arteries on either side of the neck, and in the main artery leaving the heart). In these locations blood might be expected to be well oxygenated. Central chemoreceptors are located on the surface of the medulla (brainstem) and they monitor the pH of the cerebrospinal fluid (CSF).

Alongside these changes, (rate and depth of) breathing will be affected, causing more efficient release of carbon dioxide and intake of oxygen

Extra carbon dioxide lowers the pH of the blood plasma, as it forms carbonic acid. This sets up a series of events which increase the heart rate so that the blood containing carbon dioxide is more efficiently moved away from the active muscle, and taken to the lungs to allow the carbon dioxide to be exhaled (excreted in breath leaving the body).
At the same time, extra oxygen will be sent round the body to be used in aerobic respiration by the active muscle tissue, as well as reducing any oxygen debt that has been caused by the increased activity.

Impulses from receptors detecting a rise in carbon dioxide (or a fall in oxygen) pass along nerve fibres to the cardiac centre (cardiovascular centre) within the medulla oblongata. This then sends a greater frequency of impulses to the sinoatrial node of the heart. These impulses travel along fibres of the sympathetic nervous system (a section of the autonomic nervous system, itself part of the peripheral nervous system). The 'accelerator nerve' releases noradrenaline (norepinephrine) at the neuromuscular junction.

Baroreceptors - pressure receptors - (also in carotid bodies/arteries/aorta) detect changes in blood pressure as a result of increased heart activity. In a sense they operate in a different way to chemoreceptors. They detect stretching of blood vessels and act to limit the increase in blood pressure. They send impulses to another region of the cardiac centre (cardiovascular centre). This causes an increased frequency of nerve impulses via the parasympathetic nervous system (another - separate - section of the autonomic nervous system). These pass via a branch of the vagus nerve [CNX - the 10th cranial nerve], and its normal frequency of impulses slightly inhibits the sinoatrial node - 'the vagal brake' - restricting the resting heart rate to 60-80 (Hz). This 'inhibitory nerve' releases acetylcholine at the neuromuscular junction

The sympathetic and parasympathetic nervous systems act in opposition to one another, so stimulation of one inhibits the other.

heart_carotid_and_aortic_bodies (46K)
Location of receptors in relation to heart

Within the heart

The heart is mainly composed of muscle tissue, with well-defined sheets of connective tissue giving it some shape, as well as providing electrical insulation between the main sections of the heart (upper atria and lower ventricles, left and right sides). It is important that the atria and ventricles have distinct phases of diastole (relaxation, allowing the chamber to fill with blood) followed by systole (contraction, moving blood onwards and out), and that each contraction is synchronised.

Muscle tissue is electrically polarised like neurones (negative membrane potential). Its contraction is stimulated by the opening of 'fast' ion channels which allow the entry of sodium ions, which depolarise the membrane. Then potassium ion channels open, causing efflux of potassium ions as well as opening of calcium ion channels which allow calcium ions in. This causes the internal release of calcium ions from the sarcoplasmic reticulum which starts the interaction of proteins causing muscle fibres to shorten. As in neurones, potassium ion channnels close but small amounts of potassium ions continue to leak out, resulting in repolarisation of the membrane. Cardiac muscle cells are in contact with one another by means of structures called gap junctions which ensure that contractions are co-ordinated.

Certain bodies within the heart are composed of specialised cells which generate impulses but do not really contract. Each has a different frequency but this can be overridden by other impulses.
These make up the heart's electrical conduction system, shown in yellow on the diagram on the right.

The sinoatrial node (SAN) is the main pacemaker of the heart, and it is located in the wall of the right atrium, between the superior and inferior venae cavae (upper and lower main veins of the body). Extending out from this are a series of specialised muscle fibres [not nerve fibres] - the 'electrical conduction system' - which carry action potentials to various sections of the heart. These waves of electrical activity spread across the atria. Distinct fibres ('Bachman's bundle') extend straight across the left atrium, and others loop around the right atrium, so the atria contract together. However the impulses do not pass directly from the atria to the ventricles, and the atria are able to empty their contents smoothly into the ventricles. The action potentials reach another section of specialised tissue - the atrioventricular node (AVN) in the centre of the heart between the atria and the ventricles. This sends the action potentials onwards via bundles of His which pass down on either side of the septum between the ventricles and continue into much-branching Purkyne fibres or tissue which spread out over the ventricles. These cause contraction of the ventricles, beginning at the base and extending upwards, so that blood is effectively pushed upwards and out. The right ventricle sends deoxygenated blood towards the lungs, and the left ventricle sends blood out via the aorta to all parts of the body.

Hearts don't just beat faster ...

In addition to increased heart rate, exercise will cause the heart to beat more deeply. In other words each heartbeat will cause a greater volume of blood to flow. The stroke volume is the the volume of blood pumped out (from the left ventricle) per beat. Stroke volume cannot usually be directly measured, but it may be calculated from difference in the volume of blood before and after the heart beats - usually obtained using an echocardiogram - output from a (3-D) scanner using ultrasound. These values are also called the end-systolic volume and the end-diastolic volume.

A typical value for stroke volume is 70 cm3 in a healthy man (70kg). With a normal heart rate of 70 beats per minute this gives the volume of blood pumped per minute as 4900 cm3, which is quite close to the total amount of blood in the body.

Cardiac output (CO) may be calculated using the formula CO = R V, where R= heart rate and V= stroke volume.

Starling's law states that the stroke volume of the heart will increase in response to an increase in the volume of blood in the ventricles, before contraction (the end-diastolic volume) - sometimes known as preload. Effectively the inflow of extra blood stretches the cardiac muscle fibres, leading to an increase in the force of contraction.
SAN_AVN_etc (258K)
The electrical conduction system of the heart

ECG starting with P

heartPQRST (15K)
ecg (10K)
The electrocardiogram trace is typically labelled PQRS. It can be used to monitor the process of electrical stimulation of the heart chambers. This is achieved by attaching electrodes to the skin of the chest, arms, and legs. The waveform is not like membrane potential, and it does not cross over (much) from negative to positive. The regularity and timings of the peaks of the waveform can give insight into the condition of the heart, and it is also expressed as a number of strangely labelled sections such as the PR segment and PR interval.

The P wave is caused by depolarisation of the atria, as initiated by the sino-atrial node.

The QRS section is caused by depolarisation of the ventricles, as a result of electrical activity passing down from the atrioventricular node, and it shows as a long sharp spike at R. This shows that ventricular contraction is strong and quick.

The T wave represents ventricular repolarisation.
Atrial repolarisation is hidden by QRS.

Related topics

This topic leads on fairly logically to a series of related topics (none of them exactly trivial):

cardiac arrhythmias
artificial pacemakers
heart attacks myocardial infarctions
beta blockers
cardioactive drugs

Other related topics on this site

(also accessible from the drop-down menu above)

Receptors - similar level - more detail about Pacinian corpuscles, retina cells
Nerve cells, Nerve impulses - similar level - more detail about impulses, Schwann cells etc
Synaptic transmission - similar level - more detail about types of synapse, neurotransmitter substances, temporal and spatial summation, effects of specific drugs

The heart and circulation - simpler, more general

Web references

Cardiovascular System (Heart)

Jan Evangelista Purkyne From Wikipedia, the free encyclopedia - helpfully telling us how to say his name [if talking in Czech], and reminding us of several things named after him, including Purkinje cells, large neurons with many branching dendrites found in the cerebellum.

Components of the ECG Strip

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